The present invention relates to two way wireless communication systems. In particular, the present invention relates to wireless telephone systems with diversity signal transmission for reducing signal fading and measuring subscriber location.
Wireless radio communication is subject to the adverse effects of signal fading, in which the signal level at the receiver temporarily loses strength for a variety of reasons, such as from variable multipath reflections causing signal cancellation, time varying transmission loss due to atmospheric conditions, and mobile receiver movement introducing obstructions into the signal path, and the like. Signal fading causes poor reception, inconvenience, or in extreme cases, a loss of call connection.
It is known to use various forms of signal diversity to reduce fading. For example, as indicated in U.S. Pat. No. 5,280,472, signal diversity mitigates the deleterious effects of fading. There are three major types of diversity: time diversity, frequency diversity and space diversity.
Time diversity is obtained by the use of repetition, interleaving or error correction coding, which is a form of repetition. Error detection techniques in combination with automatic retransmission, provide a form of time diversity.
In frequency diversity, signal energy is spread over a wide bandwidth to combat fading. Frequency modulation (FM) is a form of frequency diversity. Another form of frequency diversity is code division multiple access (CDMA) also know as spread spectrum. Due to its inherent nature as a wideband signal, CDMA is less susceptible to fading as compared to a narrow band modulation signal. Since fading generally occurs in only a portion of the radio spectrum at any one given time, a spread spectrum signal is inherently resistant to the adverse effects of fading.
Space diversity is provided by transmitting or receiving the same signal on more than one geographically separated antennas. Space diversity provides alternate signal paths to guard against any one path being subject to fading at any one time. Space diversity also creates some time diversity since the receiver receives the same signal separated by small propagation delays. The difference in propagation delay requires that the receiver be able to discriminate between the arriving signals. One solution is to use multiple receivers, one for each arriving signal. For instance, it is known from U.S. Pat. No. 5,280,472 to deliberately introduce relatively small delays compared to an information symbol, into a space diversity multiple antenna CDMA system in order to create artificial multipath time diversity signals greater than one chip delay up to a few chips. CDMA systems are capable of discriminating between identical plural signals arriving at the receiver with different propagation delays greater than one chip delay. Such receivers are known as Rake receivers. However, prior art systems require multiple CDMA receivers, one CDMA receiver for each separate received CDMA signal. It is desirable to provide a system for receiving time diversity CDMA signals which does not require multiple CDMA receivers.
Measuring or determining the location of mobile units is well known. In some systems, fixed antennas measure the mobile location. In other systems, the mobile unit determines its location from multiple received signals. If the system is two way, the communication link permits both the mobile subscriber and the fixed system to exchange location data. Various known systems use satellites or multiple antennas to provide information on the location of a mobile subscriber. For example, multiple directional receiving antennas can be used to triangulate the position of a mobile transmitter. In such systems, the stationary receivers determine the mobile subscriber location; in other systems, the mobile subscriber determines its location from the received signals. For example, the Global Position System (GPS) is a multiple satellite system providing signals which permit a mobile subscriber station to determine its position in latitude and longitude. However, both satellite systems and the GPS receivers for receiving satellite signals tend to be expensive.
The combination of a GPS receiver and a cellular telephone is shown in U.S. Pat. No. 5,223,844. Such combination provides useful services, as for example a security alarm service to deter car theft, in which tripping the alarm also alerts the security service to the location of the car. Generally, it is desirable to provide a system which combines telephone or data service with location measurement at a reasonable cost.
It is desirable to provide a system of time diversity signals using time division multiple access (TDMA) in various combinations with CDMA and space diversity antennas, to provide a variety of systems which resist fading, reduce receiver cost, and provide location measurement for mobile subscribers.
The present invention is embodied in a wireless communication system in which diversity is used to reduce fading and simplify receiver design. The present invention is further embodied in a wireless communication system in which time division signals are code division (spread spectrum) multiplexed onto space diverse antennas to provide a wireless communication system with the ability to determine subscriber location using the same communication signals which are used forte primary wireless communication.
A subscriber station for a CDMA cellular system is provided that utilizes transmission diversity by transmitting a plurality of PN spread spectrum signals. Data is transmitted as PN spread spectrum modulated data signals where at least a first PN spread spectrum modulated data signal is formed by spreading and modulating the data with a first PN code and a second PN spread spectrum modulated data signal is formed by spreading and modulating the data with a second PN code which is different than the first PN code. The subscriber station has a receiver which receives the first and second PN spread spectrum modulated data signals. A despreader and demodulator despreads and demodulates the received PN spread spectrum modulated data signal using the respective PN codes. A combiner reconstructs the data from the despread and demodulated data signals. The PN spread spectrum modulated data signals are preferably transmitted in different time slots so that the receiver is configured to receive the PN spread spectrum modulated data signals in the respective slots. Preferably, three PN spread spectrum modulated data signal are transmitted each from a different spaced apart antenna so that the subscriber station includes a measuring device for measuring respective times of arrival of the three data signals. A processor computes the location of the subscriber station from respective measured times of arrival of the data signals.
Specifically, a data packet which for example may carry telephone voice traffic, is transmitted at three different times from three different antennas. The receiver thus receives the same data packet at three different times from three different antennas. The receiver uses the best data packet or combination of the data packets to reduce the effects of fading.
In addition, the receiver uses the absolute and extrapolated relative time of arrival of the three data packets to determine its location from the three transmitting antennas. First, absolute range to one antenna is determined by the time required for a round trip message. Then, the relative time of arrival of data packets, referenced to a universal time, from the two other antennas indicates the relative distances as compared to the first antenna. Since all three transmitting antennas are at known fixed locations, the receiver computes its own location as the intersection of three constant distance curves (in the two dimensional case, circles, or in the three dimensional case, the intersection of three spheres). In the alternative, the mobile subscriber station provides raw delay measurement data back to a fixed station, or location service center, which computes the mobile subscriber location.
More particularly, the present invention is embodied in a system using CDMA to modulate a TDMA signal which is transmitted from three space diversity antennas. In a first embodiment, the TDMA signals are used to transmit multiple repetitions of the same data packet from a transfer station with three space diversity antennas. In a second embodiment, the TDMA signals are used to transmit multiple repetitions of the same data packet from three transfer stations each transfer station including one of the three space diversity antennas. The data packets could either be identical, or could carry substantially the same information, but modulated with different spreading codes or different segments of the same spreading code.